14 Feb 11
So I know if I blink my eyes as fast as I can, I am making 100 millisecond clips of time. So I blink my eyes like that at my overhead fan, and I notice I can stop the blades ala strobe effect. However, the blades are not crisp images like those strobes which flash for a few microseconds, my eyes are open for 100 millisecond clips. So in that 100 ms, the blade, which is normally 150 mm wide, is blurry, looks twice as wide. So with that info, how fast is the blade turning?
14 Feb 11
3 edits
I don't know about you, but I certainly can't blink my eyes 10 times/second. Therefore I think the question as posed is accurate.
If you *could* blink your eyes 10x/second, and assuming the fan has 5 blades, it would be spinning approximately 2 RPM.
*edit* I re-read the post, and your question may be intended differently than I read it.
If the fan blade moves 150mm in 100ms, then the linear velocity of that section of fan blade is 1.5m/s. It is not possible to calculate the rotational velocity of the fan with the given information.
14 Feb 11
Originally posted by forkedknighti told you, it is the hamster!
I don't know about you, but I certainly can't blink my eyes 10 times/second. Therefore I think the question as posed is accurate.
If you *could* blink your eyes 10x/second, and assuming the fan has 5 blades, it would be spinning approximately 2 RPM.
*edit* I re-read the post, and your question may be intended differently than I read it.
If the ...[text shortened]... It is not possible to calculate the rotational velocity of the fan with the given information.
15 Feb 11
1 edit
Originally posted by forkedknightI got that after I posted. The diameter of the fan blades is 1.3 meters. You only need one blade to make the calculation.
I don't know about you, but I certainly can't blink my eyes 10 times/second. Therefore I think the question as posed is accurate.
If you *could* blink your eyes 10x/second, and assuming the fan has 5 blades, it would be spinning approximately 2 RPM.
*edit* I re-read the post, and your question may be intended differently than I read it.
If the ...[text shortened]... It is not possible to calculate the rotational velocity of the fan with the given information.
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15 Feb 11
2 edits
Originally posted by sonhouseIn 100 milliseconds the blade tip moves 150 mm
So I know if I blink my eyes as fast as I can, I am making 100 millisecond clips of time. So I blink my eyes like that at my overhead fan, and I notice I can stop the blades ala strobe effect. However, the blades are not crisp images like those strobes which flash for a few microseconds, my eyes are open for 100 millisecond clips. So in that 100 ms, the bla ...[text shortened]... is blurry, looks twice as wide. So with that info, how fast is the blade turning?so it is moving
so, converting to metres and seconds:
Velocity of tip = 0.15 / 0.1 m/s = 1.5 m/s
blade diameter = 1.3 metres
blade circumference = 1.3 * pi metres
so tip will travel circumference in
1.3 * pi / 1.5 seconds
and the rotational speed is 1.5 /( 1.3 * pi) s^-1
= 0.367 rotations per second, a slowly rotating fan.
The extra constraint is that the blades must appear stopped.
You are could be closing your eyes for 2.6 seconds, which is what would be needed for the first blade to come round again (( 1.3 * pi - 0.15)/1.5 = 2.6), but that sounds a bit long...
So perhaps it is a 4 bladed fan and you are closing your eyes for about 0.66 seconds between snapshots?
15 Feb 11
1 edit
Originally posted by iamatigerYes, it is a four bladed fan. Using the tip size of 1.3 meters, I got about 45 RPM.
In 100 milliseconds the blade tip moves 150 mm
so, converting to metres and seconds:
Velocity of tip = 0.15 / 0.1 m/s = 1.5 m/s
blade diameter = 1.3 metres
blade circumference = 1.3 * pi metres
so tip will travel circumference in
1.3 * pi / 1.5 seconds
and the rotational speed is 1.5 /( 1.3 * pi) s^-1
= 0.367 rotations per second, ...[text shortened]... aps it is a 4 bladed fan and you are closing your eyes for about 0.66 seconds between snapshots?
I reasoned the blade as seen is blurred to twice its real size, or 300 mm, so covering twice the distance in 100 ms. In order to get the exact speed with a strobe, you would have to have two microsecond strobe pulses spaced 100 ms apart to give two blade widths because one strobe pulse will only show you the position of a single blade at a single time. You can put some marker on the blades that allows the marked blade to be picked out by the strobe pulse, stopping that blade at the same place in its revolution, then simply counting the PRF but you don't get that luxury when simply blinking your eyes at some rate, I read when eyes blink, it is on the order of 100 ms of dark time when the eye closes so I reasoned if you could do that fast enough to stop slowly rotating blades, the perceived blurring width could be used to calculate the RPM's within certain limits of course.
15 Feb 11
Originally posted by sonhouseyou seem to spend a lot of time looking at that fan.
Yes, it is a four bladed fan. Using the tip size of 1.3 meters, I got about 45 RPM.
I reasoned the blade as seen is blurred to twice its real size, or 300 mm, so covering twice the distance in 100 ms. In order to get the exact speed with a strobe, you would have to have two microsecond strobe pulses spaced 100 ms apart to give two blade widths because one ...[text shortened]... perceived blurring width could be used to calculate the RPM's within certain limits of course.
you might enjoy this:
now, giving the calculated speed and a cord length of 0.5m, and the weight of the cat of around 5kg, how much force has the wall be able to withstand?
🙂
15 Feb 11
Originally posted by tharkeshOMG that cat was is serious trouble. My guess is it wouldn't do that again🙂
you seem to spend a lot of time looking at that fan.
you might enjoy this:
http://www.youtube.com/watch?v=lrI7mHVHlEc
now, giving the calculated speed and a cord length of 0.5m, and the weight of the cat of around 5kg, how much force has the wall be able to withstand?
🙂
I guess I am a fan of fan's🙂
15 Feb 11
1 edit
Originally posted by sonhouseIf the blade "appears" to be 300mm, it just means that the front edge is being replaced by the back edge, so the distance traveled is only a single width of the blade, or 150mm.
Yes, it is a four bladed fan. Using the tip size of 1.3 meters, I got about 45 RPM.
I reasoned the blade as seen is blurred to twice its real size, or 300 mm, so covering twice the distance in 100 ms. In order to get the exact speed with a strobe, you would have to have two microsecond strobe pulses spaced 100 ms apart to give two blade widths because one perceived blurring width could be used to calculate the RPM's within certain limits of course.
I concur with iamatiger's estimate of .367 rps which translates to ~22 RPM
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16 Feb 11
2 edits
Originally posted by sonhouseIf you did that with the strobe, it wouldn't appear stopped, the blades would appear to be advancing one blade width every flash, unless the blades were only separated by a blades width, but that would be a lot of blades.
Yes, it is a four bladed fan. Using the tip size of 1.3 meters, I got about 45 RPM.
I reasoned the blade as seen is blurred to twice its real size, or 300 mm, so covering twice the distance in 100 ms. In order to get the exact speed with a strobe, you would have to have two microsecond strobe pulses spaced 100 ms apart to give two blade widths because one perceived blurring width could be used to calculate the RPM's within certain limits of course.
18 Feb 11
Originally posted by iamatigerThat would depend on the pulse rate. For instance, you could set up the strobe to do two quick pulses just the right time followed by a longer period, two pulses in a row with a time gap then two more pulses, you could show the same blade one blade width apart. Having said that, that is not the standard way a strobe works, it only has one pulse per period. I suggest two pulses per period would do the trick.
If you did that with the strobe, it wouldn't appear stopped, the blades would appear to be advancing one blade width every flash, unless the blades were only separated by a blades width, but that would be a lot of blades.